What car(s) get excellent gas mileage, and also have enough room to sleep in? (e.g. Prius hatchback, Honda Fit?

I'm looking for a car to go on a long road trip, and I am looking to save money on gas and avoid hotels and sleep in my car when possible. Some cars I have considered are the Honda Fit, Prius hatchback and Scion xd (the seats fold flat and provide enough room to sleep in. I've heard that when going over 55 mph the Prius doesn't get very good gas mileage at all. I expect to be going at least 70 most of the way. Any info at all will be greatly appreciated